1. Field of Invention
The present invention relates to a DC motor. More particularly, the present invention relates to a constant-current driving method and circuit for driving a DC motor.
2. Description of Related Art
Traditionally, the use of a DC fan incorporating a DC brushless motor has been for scattering heat in various electronic products. This is because when heat accumulates within the products and cannot be dispersed, components of the products cannot function and operate well, and the whole system may even fail or be permanently damaged. Therefore, a DC fan incorporating a DC brushless motor acts as a heat-scattering device to make various components in the system operate normally in a better temperature environment. Due to different factors including heat scattering, high rotation speed has been preferred in the design of a DC motor.
Moreover, different fields of application and required qualities of the electronic product make two other design requirements for a DC motor gradually more important. One of the two requirements is low noise, and the other is low power operation. Low power operation means low rotation speed and low current. A DC fan with low rotation speed and low current can be used in many applications, such as blowing an aromatic around or spreading moisture in a humidifier.
The traditional method for driving a DC motor of a fan is a constant-voltage-output or constant-voltage driving method. FIG. 1 illustrates a driving circuit, which includes two transistors and two rotatable armature windings of a DC motor. Transistors 102 and 106 are bipolar junction transistors (BJT), and their collector-to-emitter voltages Vce are Vce1 and Vce2, respectively, which are the voltages at the two output nodes DO and DOB in FIG. 1. The resistances R of the two armature windings 100 and 104 are R1 and R2, respectively, the supply voltage and total supply current are Vcc and Icc, respectively, and the currents I flowing through the two armature windings 100 and 104 are I1 and I2, respectively.
When the constant-voltage driving is used, the two transistors 102 and 106 are controlled by two control voltage signals V1 and V2 and do not turn on simultaneously. When each transistor is turned off, it enters the cutoff region, and the current I is very small and close to zero. Therefore, the voltage at the output node is approximately equal to Vcc. When each transistor is turned on, it may gradually enter and remain in the saturation region, in which the collector-to-emitter saturation voltage is Vce1,sat for the transistor 102 and is Vce2,sat for the transistor 106, both values of which are specified on the transistor data sheet. Because the output voltage Vce of a transistor is almost constant when remaining in the saturation region, the driving method is called constant-voltage-output driving. At this moment,                     Vcc        =                              V                          ce1              ,              sat                                +          I1R1                                                  =                                    V                              ce2                ,                sat                                      +            I2R2                          ,            in which R1 and R2 are constant and determined by the two armature windings 100 and 104. When Vcc is varied over a substantial range (0 to 12V), the two saturation voltages Vce1,sat and Vce2,sat vary over a small range (0.3 to 0.6V), and the currents I1 and I2 vary over a relatively large range (0 mA to the maximum allowed current value). Even 1.5 to 2 times of the original current value can occur when the transistor is locked in the saturation region. Therefore, the constant-voltage driving is suitable for manufacturing high-rotation-speed and high-current fans.
If the DC fan can be started at or above a certain driving current (current through the winding), this driving current level is called the starting current, and the magnitude of Vcc required for achieving the starting current is called the starting voltage. Since the currents I1 and I2 are inversely proportional to the resistances R1 and R2, respectively, the constant-voltage driving requires a relatively large resistance R to limit the current value if a fan with a low current and low rotation speed is to be developed. When the resistance R is large, the starting voltage required for achieving the starting current becomes large. Therefore, when the constant-voltage driving is used, the resistance R affects the starting voltage, and that means the starting voltage is more sensitive to manufacturing differences causing different values of the resistance R. In addition, since high resistance current-limiting windings are not easy to produce, impedance or resistance matching of the windings is difficult for a low-rotation-speed fan. Furthermore, the rotation speed is often not consistent between batches of low-rotation-speed fans produced, that is, the rotation speed is not stable and is poorly controlled.
For the foregoing reasons, the constant-voltage driving method is not suitable for making low-current and low-rotation-speed fans, and there is a need for a better driving method.